1. Field of the Invention
The present invention relates to the field of ultrasonic equipment and, more specifically, systems for the transmission of acoustic energy into liquid media during acoustic cavitation-based sonochemical and sonomechanical processes.
2. Description of the Related Art
Advantages of using ultrasonically induced acoustic cavitation to carry out technological processes in liquids are well documented, for example, in the following references: K. S. Suslick, Sonochemistry, Science 247, pp. 1439-1445 (1990); T. J. Mason, Practical Sonochemistry, A User's Guide to Applications in Chemistry and Chemical Engineering, Ellis Norwood Publishers, West Sussex, England (1991), hereby incorporated by reference.
In the prior art ultrasonic systems designed for industrial sonochemical and sonomechanical processes, the liquid commonly is subjected to ultrasonic treatment as it flows through a reactor. The latter commonly consists of a reactor chamber incorporating an ultrasonic waveguide radiator (horn) connected to an electro-acoustical transducer. The horn is used to amplify the transducer's vibration amplitude, which is necessary because the vibration amplitude of the transducer itself is not sufficient for most industrial processes. Such ultrasonic reactor systems are described, for example, in U.S. Published Patent Application No. 2006/0196915, U.S. Published Patent Application No. 2005/0274600 and U.S. Pat. No. 7,157,058, hereby incorporated by reference.
All of the abovementioned systems possess an important common drawback, which restricts their ability to create powerful ultrasonic cavitation fields and limits their production capacity. This drawback stems from the fact that the acoustic horns used in the prior art generally have tapered shapes, such as conical, exponential, catenoidal, stepped, or more complex, converging in the direction of the load. While these horns may have high gain factors and permit significantly increasing vibration amplitudes, the increase occurs always at the expense of the output surface areas, which become small as a result. Therefore, while converging horns are capable of increasing the specific acoustic power (or vibration amplitude at a given ultrasonic frequency) radiated by a transducer into a load quite effectively, they do not permit achieving significant levels of total radiated acoustic power. The total power provided by a generator and a transducer is, therefore, not efficiently transmitted into the liquid (reflected back). Consequentially, sonochemical reactors based on these horns are effective only on the laboratory scale. Success of industrial applications of such systems is limited. Additionally, in the design of the abovementioned ultrasonic reactors, the size and shape of the cavitation field itself is not taken into account, which further lowers their efficiency.
In the work by G. Cervant, J.-L. Laborde, et al., “Spatio-Temporal Dynamics of Cavitation Bubble Clouds in a Low Frequency Reactor,” Ultrasonic Sonochemistry 8 (2001), 163-174, hereby incorporated by reference, a theoretical study describing the shape, size and position of the cavitation field formed under an ultrasonic radiator is described in detail. In the article by A. Moussatov, R Mettin, C. Granger et all “Evolution of Acoustic Cavitation Structures Near Larger Emitting Surface”, WCU 2003, Paris, Sep. 7-10, 2003, hereby incorporated by reference, a similar experimental study was conducted. The results show that during operation of an acoustic horn, a stable well developed cavitation filed only starts to form when the following two necessary conditions are fulfilled: (1) specific intensity of the ultrasonic energy radiated into liquid exceeds 8 W/cm2 (for water) and (2) the output diameter of the radiator's cross section is on the order of the acoustic wavelength, λ, in the original supplied liquid load (before cavitation has started). In other words, the radiator should transmit a planar acoustic wave into the liquid. In this case, the cavitation field starts to become stable and takes the shape of an upside-down circular cone. It is important to also point out that such stable cavitation field at the described conditions has maximum possible geometrical size. Therefore, only if such stable cavitation field can be established in an ultrasonic reactor will the productivity be maximized and will the optimal stability and the operational quality be reached. The exact size of the cavitation field formed under an ultrasonic radiator was not, however, obtained in the abovementioned studies. Additionally, cavitation formed near the lateral surface of the radiator was not studied.
Deposition of at least 8 W/cm2 (for water) of specific acoustic power requires the amplitudes of vibration velocity of the output surface of an acoustic horn to exceed 112 cm/sec (rms) (oscillatory amplitudes exceeding 25 microns peak-to-peak at 20 kHz). Since most materials used to make ultrasonic transducers cannot themselves provide such amplitudes, ultrasonic horns must be utilized, having gain factors of at least 3. Even higher horn gain factors are preferred because most sonochemical or sonomechanical processes require amplitudes that are much greater than this threshold value. Since the speed of sound in most liquids of interest, such as water, oils, alcohols, etc, is on the order of 1500 msec, λ in those liquids at the common working ultrasonic frequencies of 18-22 kHz is about 65-75 mm. As mentioned above, it is necessary that the diameter of the output surface of the horn be close to λ in the liquid load. Consequentially, only the horns that provide high output oscillatory amplitudes (high gain factors) and have large output surface areas simultaneously are truly appropriate for the use in efficient high-capacity industrial ultrasonic reactor systems for sonomechanical and sonochemical processes. None of the common converging horns are, therefore, appropriate.
A prior art “Barbell Horn” design, U.S. Pat. No. 7,156,201, hereby incorporated by reference, circumvents the abovementioned limitation of converging horns to a large degree, being able to provide high output oscillatory amplitudes (high gain factors) and large output surface areas simultaneously. In the same prior art, a modified version of the Barbell Horn is also introduced, which may be called “Long Barbell Horn.” This horn has a very large lateral radiation surface and is also convenient for the use in the efficient high-capacity industrial ultrasonic reactor systems.
The prior art “Barbell Horn”, its derivatives as well as the related ultrasonic reactor designs, however, are subject to some important limitations. U.S. Pat. No. 7,156,201 provides a system of equations that is suitable only for the calculation of the Barbell Horns with cone-shaped transitional sections (parts of the horns that have changing cross-sections). Additionally, a restriction exists in the description and in the claims of the same prior art, requiring that the length of any transitional section be equal or greater than Log(N)/k, where k=ω/C is the wave number for the transitional section, N is the ratio of the diameters of the thick and the thin cylindrical sections that are adjacent to the transitional section, ω is the angular vibration frequency, C is the sound velocity in the horn material at the transitional section (with phase velocity dispersion taken into account). This restriction came from the fact that the specified length of the transitional section is critical from the standpoint of the passage of a longitudinal acoustic wave. Such selection of the length of the transitional section was thought to be necessary to decrease the degree of dynamical strain and stress along the section length and thus to increase the operational life of the waveguide-radiator. The design principles and the calculation method for the horns which are free from this restriction were not available and are not provided in the prior art.
Additionally, the only ultrasonic reactor designs mentioned in the prior art are those based on the Barbell Horns equipped with additional resonance elements, such as vibrating disks, spheres, helical surfaces, etc. All these additional elements significantly complicate the construction of the Barbell Horns, introduce additional mechanical connections and, therefore, reduce life span and reliability. It is also clear that utilizing the Barbell Horns or any of their modified versions in a non-restricted or an incorrectly restricted volume (reactor chamber) leads to an inefficient process, since not all liquid is put through the well developed cavitation field zone and/or the optimal treatment time in the cavitation field is not reached.
Therefore, to be able to maximize the effect of the ultrasonic cavitation treatment on a liquid load (pure liquid, liquid mixture, liquid emulsion, suspension of solid particles in a liquid, polymer melts, etc.), a well defined need exists to develop: 1) improved Barbell Horn designs, free from the abovementioned limitations and 2) improved ultrasonic reactor designs in which a Barbell Horn (of a novel design introduced in this invention or of a design described in the prior art) is correctly placed inside a flow-through (or stationary) volume (also called reactor chamber, flow cell, etc.).